Particle beam devices for analyzing and/or treating an object are known from the prior art. More particularly, electron beam devices, for example scanning electron microscopes (referred to as SEM below) or transmission electron microscopes (referred to as TEM below), are known.
A particle beam generator is used in a particle beam device for generating a particle beam. By way of example, an electron beam generator is used in a TEM for generating an electron beam. In order to obtain a high resolution, it is known to limit chromatic errors generated by components of the TEM. To this end, the energy width of the electrons of the electron beam is reduced in the prior art. More precisely, the electrons of the electron beam, which emerge from the electron beam generator, are filtered using a monochromator unit. The monochromator unit only lets those electrons of the electron beam pass into the further beam path of the TEM that only have a small deviation from a predeterminable energy. The electrons, which are filtered using the monochromator unit and subsequently enter the further beam path of the TEM, are then accelerated using an acceleration unit situated downstream from the monochromator unit.
The prior art has disclosed a three-part design of an electron beam generator for a TEM. Thus, this known electron beam generator comprises an electron emitter, a suppressor electrode and an extractor electrode. Moreover, the TEM has an electrode unit that is connected between the electron beam generator and the monochromator unit. The electrode unit is used to focus an electron beam, which was generated by the electrode beam generator, into a specific geometrically fixed plane of the monochromator unit. Hence the electrode unit acts in a focusing fashion. Furthermore, it likewise has a three-part design. Thus, the electrode unit has a first electrode apparatus, a second electrode apparatus and a third electrode apparatus, wherein the second electrode apparatus is connected between the first electrode apparatus and the third electrode apparatus. The first electrode apparatus is embodied as the extractor electrode of the electron beam generator. Hence the extractor electrode is both a component of the electron beam generator and a component of the electrode unit. In the known electrode unit, the third electrode apparatus is embodied in the form of an exit electrode. The exit electrode is both a component of the electrode unit and a first component of the monochromator unit. In this design of the electrode unit, both the extractor electrode and the exit electrode lie at the potential of the extractor electrode (also called extractor potential).
Electrons of the electron beam pass through the monochromator unit along a non-straight axis of the monochromator unit. The electrons respectively form a crossover at one or more points within the monochromator unit. Above, and also below, a crossover is understood to be a position on an axis, e.g. the optical axis of a particle beam device, at which the particles (the electrons in the case of a TEM) emitted by the particle emitter (e.g. the electron beam generator) converge and a cross-sectional area of the particle beam accordingly has a local minimum. In order to be able, along the optical axis of the TEM, to select, in respect of type and design, the components, e.g. electrode and acceleration units, that are arranged downstream or upstream of the monochromator unit in respect of the beam path of the electrons, it is sufficient to know the position (location) of a crossover on the input side of the monochromator unit and the position (location) of a crossover on the output side of the monochromator unit. The input side of the monochromator unit is the side from which electrons enter into the monochromator unit. The output side of the monochromator unit is the side from which electrons exit the monochromator unit. Here, the crossover on the input side and the crossover on the output side of the monochromator unit lie on an axis of a straight-line equivalent beam path of the monochromator unit. Here, the straight-line equivalent beam path does not run along the actual non-straight axis of the monochromator unit but rather is the beam path that the electrons would pass through if there were no monochromator unit but the electrons were to experience the same effect as in the monochromator unit. The crossover on the input side and the crossover on the output side of the monochromator unit are virtual.
The electrode unit serves to focus an electron beam, generated by the electron beam generator, onto a specific geometrically fixed plane of the crossover on the input side of the monochromator unit. In order to bring this about, the prior art has disclosed the practice of applying a specific potential to the second electrode apparatus for a predetermined extractor potential at the extractor electrode. After the electrons have exited the monochromator unit, the electrons are accelerated to a desired energy in an acceleration unit in the further beam path of the TEM. As a result of the potential profile in the acceleration unit the acceleration unit has a fixed focusing effect for a specific electron energy. It is for this reason that the virtual crossover on the output side of the monochromator unit (a first crossover) is imaged at a specific position on the optical axis of the TEM after the electron beam passes through the acceleration unit. There is a real second crossover at this specific position, namely the image of the virtual crossover on the output side of the monochromator unit. In the prior art, the position of the second crossover is prescribed by a selected high voltage, by which the desired energy of the electrons is obtained, and by a predetermined extractor voltage. If the high voltage changes (i.e. if the desired energy of the electrons changes) and/or if the extractor voltage changes, the position of the second crossover also changes.
It is known that the focusing effect of the acceleration unit is determined firstly by the potential drop between an exit electrode, lying at the extractor potential, of the monochromator unit and a first acceleration electrode of the acceleration unit and secondly by the potential drop between the first acceleration electrode and a second acceleration electrode in the acceleration unit. However, the potential drop between the exit electrode, lying at the extractor potential, of the monochromator unit and the first acceleration electrode of the acceleration unit mainly contributes to the focusing effect of the acceleration unit. This has to do with the fact that the relative increase in the electron energy (with respect to the energy at the extractor electrode) when the electrons pass through the path between the extractor electrode and the acceleration unit is greatest between the extractor electrode and the first acceleration electrode. Here, the relative increase in energy is understood to mean a change in energy between two of the aforementioned electrodes with respect to the energy at a first electrode.
The position of the second crossover depends on the fixing of the extractor potential, on the first acceleration potential applied to the first acceleration electrode (i.e. also on the high voltage determining the energy of the electrons) and on the second acceleration potential applied to the second acceleration electrode. Different positions of the second crossover on the optical axis emerge depending on the extractor potential and depending on the high voltage determining the energy of the electrons. Hence, these different positions are fixedly prescribed as a result of the selected extractor potential and the high voltage determining the electron energy and cannot be varied.
In respect of the aforementioned prior art, reference is made to DE 196 33 496 A1, U.S. Pat. No. 6,495,826 B2 and EP 1 277 221 B1, which are incorporated herein by reference.
Accordingly, it would be desirable to specify a particle beam device and a method, in which the position of a crossover on an optical axis of a particle beam device can be freely adjusted, even in the case of a fixed extractor potential and a fixed high voltage.